1. Field of the Invention
The present invention relates to a switching power source device, and more particularly, to a self-excited oscillation type switching power source device provided with an overcurrent protection circuit.
2. Description of the Related Art
FIG. 14 is a circuit diagram of a switching power source device (example 1) disclosed in U.S. patent application Ser. No. 10/232,901 filed on Sep. 3, 2002.
A first switching circuit S1 and an input power source Vin are connected in series with a series circuit of a primary winding T1 and an inductor L of a transformer T.
A series circuit including a second switching circuit S2 and a capacitor C is connected in parallel to a series circuit including a primary wiring T1 and an inductor L of a transformer T.
A series circuit including a second switching circuit S2 and a capacitor is connected in parallel with the series circuit including the first switching circuit S1 and the inductor L.
A rectification-smoothing circuit including a rectification element Ds is provided for a secondary winding T2 of the transformer T.
The first switching circuit S1 is a circuit including a first switching element Q1, a first diode D1, and a first capacitor C1 connected in parallel to each other. The second switching circuit S2 is a circuit including a second switching element Q2, a second diode D2, and a second capacitor C2 connected in parallel to each other.
A switching control circuit is connected between a first driving winding T3 provided for the transformer T and the control terminal of the first switching element Q1, and also between a second driving winding T4 and the control terminal of the second switching element Q2, respectively.
The switching control circuit controls such that the first and second switching elements Q1 and Q2 are alternately turned on/off while the off-period when both of the first and second switching elements Q1 and Q2 are turned off is interposed between the on/off operations. Energy is stored in the primary winding T1 and the inductor L during the on-period of the first switching element Q1, and the energy is released during the off-period of the first switching element Q1, and thus, the first and second switching elements Q1 and Q2 are self-excited-oscillated.
Referring to the above-described configuration, the inductor L and the capacitor C constitute a resonance circuit which resonates during the off-period of the first switching element Q1.
The above-described switching control circuit includes an on-period control circuit set at a time constant at which the first switching element Q1 is turned off at a predetermined time after the first switching element Q1 is turned on, and a second on-period control circuit set at a time constant at which the second switching element Q2 is turned off, such that the resonance current flows through the series circuit, including the second switching element Q2 and the inductor L, after the second switching element Q2 is turned on and before the energy release from the second winding is completed. Thereby, the switching control circuit is operated in a continuous current mode.
Furthermore, an overcurrent protection circuit is provided. The overcurrent protection circuit includes a resistor R which is a current-detecting device connected in series with the first switching element Q1, and limits the on-period of the first switching element Q1 when the current detected by the resistor R reaches a threshold.
The operation of the overcurrent protection circuit is as follows. The transistor Tr2 is connected to the control terminal of the first switching element Q1. A voltage developed across the above-described current-detecting device (the resistor R) is applied to the control terminal of the transistor Tr2 via a resistor R6. When the current flowing through the first switching element Q1 reaches a predetermined value, the control terminal voltage of the transistor Tr2 reaches a threshold to be turned on, causing the first switching element Q1 to turn off so that the peak current flowing through the first switching element Q1 is limited. FIG. 16 shows the waveform of current Id1 flowing through first switching element Q1 which is generated when the output voltage is reduced. As seen in FIG. 16, when the output voltage is reduced, the off-period of the first switching element Q1 is kept substantially constant and the on-period is reduced. Accordingly, the switching frequency is increased, which increases the switching loss, and also, the output current is increased.
FIG. 15 shows an example of a ringing choke converter provided with a peak current limiting circuit (an example 2). When the peak current flowing through the first switching element Q1 reaches a predetermined value, the transistor Tr4 is turned on, and the first switching element Q1 is turned off.
FIG. 17 shows the waveform of current Id1, flowing through the first switching element Q1, which is generated when the output voltage is reduced. As seen in FIG. 17, as the off-period of the first switching element Q1 is increased, the switching frequency is decreased in correspondence with the reduction of the output voltage. Therefore, the increase of the switching loss is suppressed, and the output current is increased.
The above-described switching power source device and ringing choke converter have the following defects.
(1) The delay time from the time when the current flowing through the first switching element Q1 is detected and the detected voltage reaches the base-emitter threshold voltage of the transistor Tr2 to the time when the transistor Tr2 is turned on, is long. Therefore, the on-time cannot be reduced to be short, the secondary current is increased and a secondary rectification diode and so forth may be broken.
One of the reasons for this is that the transistor TR2 cannot be turned on immediately after the detection voltage reaches the base-emitter threshold voltage of the transistor Tr2. Sufficient base current is required to turn on the transistor Tr2. The time required for securing the base current is the delay time. Thus, the on-period cannot be reduced to be sufficiently short, and the output power is increased. Moreover, when the first switching element Q1 is turned off by the transistor TR2, the current flowing through the first switching element Q1 is reduced, and the voltage applied across both ends of the resistor R is reduced. When this voltage becomes less than the base-emitter threshold voltage of the transistor Tr2, the transistor Tr2 cannot be turned on. Thus, the on-speed is rapidly reduced. When the on speed of the transistor Tr2 is low, and the delay time is long, the turn-off speed of the first switching element Q1 becomes low. Thus, the switching loss is increased, and also, the on-period cannot be reduced when an overcurrent exists. The increase of the output current is more than that caused when the output voltage of the device is reduced. When the output voltage of the device is increased, inconveniences such as breaking of a primary diode and so forth are caused. Therefore, it is indispensable to rapidly turn off the first switching element Q1.
(2) When the primary peak current is limited to a predetermined value, the output power is restricted to have a substantially constant value. When the output voltage is reduced, the output current is increased. Thus, a secondary rectification diode and so forth may be broken.
(3) When the output is shortcircuited, the shortcircuit current is increased. Thus, a secondary rectification diode and so forth may be broken.
In order to overcome the problems described above, preferred embodiments of the present invention provide a self-excitation oscillation type switching power source device having an overcurrent protection circuit, in which a transistor circuit for controlling a first switching element Q1 has an improved configuration in which when the primary peak current becomes a predetermined current, the first switching element Q1 is rapidly turned off to suppress the peak current and thereby limit the output power, and when the output current of the device is increased and the output voltage is decreased, the output power is reduced, and hence, the increase of the shortcircuit current at shortcircuiting of the output can be prevented and minimized.
According to a first preferred embodiment of the present invention, a switching power source device, which carries out self-excitation oscillation, includes a primary winding T1 of a transformer T, a first switching element Q1, a current detecting device R and an input power source Vin connected in series, a rectification-smoothing circuit provided for a secondary winding T2 of the transformer T, a switching control circuit connected to a first drive winding T3 provided in the transformer T and being operable to turn on/off the first switching element Q1 to control the on-period of the first switching element Q1 and thereby control the output voltage of the device, the switching control circuit being operable to perform a control function so as to turn on a first switching device connected to the control terminal of the first switching element Q1 and thereby turn off the first switching element a predetermined period after the first switching element Q1 is turned on by a voltage developed in the first drive winding T3, the predetermined period being determined by a time constant circuit, and an overcurrent protection circuit including an on-period limiting circuit and a peak current limiting circuit, the on-period limiting circuit being operable to set the maximum on-period of the first switching element Q1 with the time constant circuit, the peak current limiting circuit including the current detection device R for detecting the peak current flowing through the first switching element Q1, a second switching device which is turned on when the detected current becomes a predetermined peak current, and a third switching device which is turned on by the turning on of the second switching device, the third switching device being connected to the control terminal of the first switching device and whereby the first switching element Q1 is turned off by the turning on of the third switching device.
Thus, the present invention provides a novel, greatly improved self-excitation oscillation type switching power source device.
According to the configuration of preferred embodiments of the present invention, when a peak current having a predetermined value is detected by the peak current limiting circuit, the second switching device is turned on, and further the second switching device is turned on, and the third switching device is caused to turn on by the turning on of the second switching device. The first switching device is turned on by the turning on of the third switching device, which causes the first switching element Q1 to turn off. Alternatively, the first switching element Q1 is turned off directly by the turning on of the third switching device and not by the turning on of the first switching device.
According to the above-described configuration, when a peak current having a predetermined value is detected, the second switching device is turned on, and then, the third switching device is turned on. Thereby, the electrical signal can be amplified, so that the first switching device is turned on, or the first switching element Q1 is turned off. Hence, when the peak current having a predetermined value is detected, the first switching element Q1 can be rapidly turned off.
Moreover, according to a second preferred embodiment of the present invention, a switching power source device which carries out self-excitation oscillation includes a series circuit including a primary winding T1 of a transformer T and an inductor L, a first switching circuit S1, a current detecting device R, and an input power source Vin connected in series with the series circuit including the primary winding T1 and the inductor L, a series circuit including a second switching circuit S2 and a capacitor C of which one end is connected to a node between the series circuit including the primary winding of the transformer T and the inductor L, the first switching circuit S1 including a parallel connection circuit that includes a first switching element Q1, a first diode D1, and a first capacitor C1, the second switching circuit S2 including a parallel connection circuit that includes a second switching element Q2, a second diode D2, and a second capacitor C2, the transformer T having a first drive winding T3 for generating a voltage which causes the first switching element Q1 to conduct, and a second drive winding T4 for generating a voltage which causes the second switching element Q2 to conduct, a rectification-smoothing circuit provided for a secondary winding of the transformer T and a switching control circuit for turning on/off the first and second switching elements Q1 and Q2 while a period in which both switching elements Q1 and Q2 are off is interposed between the on/off operations. The device preferably otherwise has the same configuration as described with respect to the above-described preferred embodiment.
In particular, in the switching power source device, the switching control circuit is controllable so as to turn on the first switching device connected to the control terminal of the first switching element Q1 and thereby turn off the first switching element at a predetermined period after the first switching element Q1 is caused to turn on by a voltage developed in the first drive winding T3, the predetermined period being determined by a time constant circuit, and an overcurrent protection circuit including an on-period limiting circuit and a peak current limiting circuit is provided, the on-period limiting circuit having the time constant circuit for setting the maximum on-period of the first switching element Q1, the peak current limiting circuit including the current detection device R for detecting the peak current flowing through the first switching element Q1, a second switching device which is turned on when the current becomes a predetermined peak current, and a third switching device which is turned on by the turning on of the second switching device, the third switching device being connected to the control terminal of the first switching device whereby the first switching element Q1 is turned off by turning on of the third switching device.
According to the above-described configuration, when a peak current having a predetermined value is detected, the second switching device is turned on, and then, the third switching device is turned on. Thereby, the electrical signal can be amplified, so that the first switching device is turned on or the first switching element Q1 is turned off. Hence, when a peak current, having a predetermined value, is detected, the first switching element Q1 can be rapidly turned off. In a two-transistor self-excitation oscillation type switching power source device, when the peak current is restricted to limit the output power, the on-period of the first switching element Q1 is decreased with the output voltage being reduced, while the off-period is kept substantially constant. Accordingly, the switching frequency is greatly improved, and the output power is significantly increased. It is especially important to detect the peak current and rapidly turn off the first switching element Q1 from the standpoints of suppressing of the output current from increasing and also suppressing of the switching loss from increasing.
Preferably, the first switching device includes a transistor, and the time constant circuit includes an impedance circuit and a capacitor to be charged and discharged which are connected to the control terminal of the transistor.
Since the first switching device includes a transistor, the charge voltage of the capacitor and the threshold value (the base-emitter voltage of for example, about 0.6 V) can be compared. Thus, the number of elements or component parts can be reduced due to this simple configuration. This serves to reduce the size and weight of the switching power source device and also the cost thereof.
Preferably, the impedance circuit includes a photocoupler for changing the impedance and controlling the on-period of the first switching element Q1, to thereby control the output voltage of the device.
Also, preferably, the impedance circuit sets the impedances at the charging and discharging of the charge/discharge capacitor in such a manner that the maximum on-period of the first switching element Q1 is decreased with the output voltage being reduced.
The charging-period of the charging/discharging capacitor is constant in the stationary state in which the input-output voltage and the load current are not changed, since the charging/discharging cycle is repeated. However, with the output voltage being reduced, the charge of the capacitor cannot be completely discharged, thereby reducing the charging period. As a result, the on-timing of the first switching device becomes earlier, and the maximum on-period of the first switching element Q1 is reduced. Thereby, the output power is reduced, so that the output current can be reduced.
Preferably, the switching control circuit has a delay circuit including a resistor or a series circuit including a resistor and a capacitor, the delay circuit being provided between the first drive winding T3 and the control terminal of the first switching element Q1. The impedance of the delay circuit is set in such a manner that when the output voltage of the device is reduced to be lower than a predetermined value, the impedance of the delay circuit prevents the first switching element Q1 from turning on, which is caused by the voltage developed in the first drive winding. The device is operated in an operation mode in which the starting and the stopping are repeated.
The delay circuit applies a voltage to the control terminal of the first switching element Q1 at a predetermined time after the voltage is developed in the first drive-winding T3. When the output voltage is reduced to be less than the predetermined voltage, the fly-back voltage generated in the first drive-winding T3 is reduced, so that turning on of the first switching element Q1 is prevented. In particular, the fly-back voltage is divided by the impedance of the delay circuit and the impedance between the control terminals of the first switching element Q1. Since the fly-back voltage is reduced, the voltage applied across the control terminals of the first switching element Q1 does not reach the threshold value. Thus, the first switching element Q1 is not turned on by the first drive-winding T3, and thus, the oscillation is stopped. Thereafter, the first switching element Q1 is turned on by the starting resistor to start, and then, is stopped. As described above, the device is operated in an oscillation mode in which the starting and the stopping are repeated. Since the starting-period is sufficiently long compared to the period taken for one cycle when continuous oscillation is carried out, the output power can be sufficiently reduced, and the output current can be significantly reduced.
Preferably, the third switching device is connected in parallel to the impedance circuit, and turns on the second switching device when the peak current becomes a predetermined peak current and subsequently turns on the third switching device to reduce the impedance of the impedance circuit, whereby the first switching element Q1 is turned off.
When the peak current becomes a predetermined peak current, the second switching device is turned on, the third switching device is turned on, the first switching device is turned on, and then the first switching device is turned off. At this time, the third switching device is driven by an electrical signal supplied when the second switching device is turned on. Accordingly, the time at which the third switching device is turned on becomes earlier. Thus, the first switching element Q1 can be rapidly turned off. Moreover, the operation of the on-period limiting circuit and that of the peak current limiting circuit can be changed continuously without skipping.
Preferably, the peak current limiting circuit inputs a voltage developed in the first drive winding T3 during the on-period of the first switching element via a resistor and a diode, the voltage being substantially proportional to the input voltage of the device.
Preferably, the peak current limiting circuit inputs the sum of a first electrical signal which increases with increases in the current flowing through the first switching element Q1 and a second electrical signal which increases with the output voltage of the device being reduced to the control terminal of the second switching device and reduces the on-period of the first switching element Q1 with increases in the input electrical signal.
When the input voltage is varied, the higher the input voltage is, the higher the overcurrent point becomes. Thus, by inputting the voltage that is developed in the first drive winding and proportional to the input voltage to the control terminal of the third switching device via the resistor and the diode, the overcurrent point can be made low only when the input voltage is high, such that the variations of the overcurrent point caused by that of the input can be minimized. In particular, when the input voltage is high, the third switching device is turned on earlier. This reduces the size and weight of the switching power source device.
Preferably, the peak current limiting circuit inputs the sum of a first electrical signal which increases with increases in the current flowing through the first switching element Q1 and a second electrical signal which increases with the output voltage of the device being reduced to the control terminal of the second switching device, and reduces the on-period of the first switching element Q1 with increases in the input electrical signal.
When the output voltage is reduced, the sum of the electrical signals input to the control terminal of the second switching device is increased. Therefore, the on-period of the first switching element Q1 is further reduced, and the output power is decreased. Thus, the output current can be reduced.
Preferably, the second electrical signal is formed in such a manner that the fly back voltage developed in the first drive-winding T3 during the off-period of the first switching element Q1 is rectified-smoothed by a diode and a capacitor. The negative potential of the capacitor and the positive potential of the first drive-winding T3 are divided by a resistor or a combination of a resistor and a Zener diode, and the divided voltage is input to the control terminal of the second switching device via the diode.
By setting the Zener diode and the voltage-dividing resistor at predetermined values according to the above-described configuration, the overcurrent characteristic curve representing the change of the output voltage with respect to the output current can have a desired shape. In particular, by setting the output voltage which is caused to be reduced by the overcurrent, to the value at which the second signal starts to increase, the second electrical signal quantity is adjusted. When the increment of the second signal with respect to one variation of the output voltage is increased, the output power is decreased as the output voltage is reduced. That is, the characteristic curve has a xe2x80x9creturning and decreasing shapexe2x80x9d. When the increment of the second electrical signal is decreased, the characteristic curve has a xe2x80x9cslightly convex shapexe2x80x9d. That is, although the output voltage is reduced, the output power is kept substantially constant. When the overcurrent characteristic curve is set so as to have an intermediate shape between the above-described characteristic curves, a xe2x80x9cvertically-changing characteristicxe2x80x9d can be obtained in which the output current is not changed although the output voltage is reduced.
Preferably, the switching control circuit includes a second on-period control circuit which controls operation so as to turn on a fourth switching device connected to the control terminal of the second switching element Q2 to turn off the second switching element Q2 a predetermined period after the second switching element Q2 is turned on by the voltage developed in the second drive winding T4, the predetermined period being determined by a time constant circuit.
In particular, the control circuit for turning off the second switching element Q2 is preferably defined in the two-transistor self-excitation oscillation switching power source device described above. Since the turn-off timing of the second switching element Q2 is determined by the time constant circuit, the on-period of the second switching element Q2 can be controlled by this simple configuration which does not require the use of an IC or other elements required in conventional devices.
Preferably, the fourth switching device includes a transistor, and the time constant circuit includes an impedance circuit and a capacitor to be charged and discharged which are connected to the control terminal of the transistor.
Accordingly, the on-period of the second switching element Q2 can be controlled by the simple configuration using a small number of parts.
Preferably, the energy stored in the primary winding T1 during the on-period of the first switching element Q1 is released from the secondary winding T2 during the off-period to produce an output.
Preferably, the fly-back type switching power source device has the unique configuration as described above.
Preferably, the time constant of the second on-period control circuit is set so that the first switching element. Q1 is turned off before the energy stored in the primary winding T1 during the on-period of the first switching element Q1 is completely released from the secondary winding T2 during the off-period, whereby the device is operated in a current continuous mode in which the current flowing through the first switching element Q1 has a trapezoidal waveform.
In particular, the second on-period control circuit forcedly interrupts the resonance current flowing through the series circuit including the second switching element Q2 and the inductor L, after the second switching element Q2 is turned on and before the energy release from the second winding is completed. That is, the second on-period control circuit is set so as to have a predetermined time constant at which the above-described operation can be carried out.
According to the above-described on-period control circuit, the second switching element Q2 is turned off to interrupt the current flowing through the inductor L before the energy release from the second winding is completed. This change in the current causes the voltage of the first winding to be inverted. Thus, a voltage is developed in the first drive-winding T3, so that the first switching element Q1 is turned on. Thereby, the device carries out a self-excitation oscillation operation, and is operated in a continuous operation mode in which the current flows in the secondary winding T2 of the transformer T, and without a stopping-period, the current flows in the primary winding. Hence, the current flowing through the first switching element Q1 at loading can be given a trapezoidal waveform. That is, the device is operated in the continuous current mode in which the waveform of the current flowing through the first switching element Q1 at heavy loading is a trapezoid. Hence, the peak value of the current flowing through the transformer T and the first switching element Q1 and the effective current can be greatly reduced. Further, the copper loss of the transformer and the conduction loss of the first switching element Q1 can be significantly reduced. Thus, the size and weight of the switching power source device can be reduced, and a high efficiency can be attained.
Preferably, the switching control circuit sets the impedances at charging and discharging of the charge-discharging capacitor of the second on-time control circuit so that the on-period of the second switching element Q2 is increased with the output voltage being reduced, whereby the first switching element Q1 is turned on after the energy stored in the primary winding T1 is released form the secondary winding T2 during the off-period, and is self-excitation-oscillated. The current flowing through the first switching element Q1 has a triangular waveform.
The peak current limiting circuit also can be applied in the case in which the current waveform is not trapezoidal but triangular. When the current waveform is triangular, the off-period is determined by the energy release period. Therefore, the switching frequency is reduced with the output voltage being decreased from the time at which the peak current limiting circuit starts to operate. When the switching frequency is reduced as described above, the output power is not increased but decreased in contrast to the case in which the switching frequency is increased. Thus, the output current is prevented from increasing. The switching loss can be reduced by turning off the first switching element Q1 more rapidly. Thus, the on-period can be reduced appropriately.
Preferably, at least one of the first and second switching elements includes a field effect transistor.
Accordingly, the parasitic capacity of the field effect transistor can be used as the capacitor C1 or the capacitor C2. Moreover, the parasitic diode of the field effect transistor can be used as the diode D1 of the diode D2. Thus, the number of parts of the device can be reduced. That is, the cost of the switching power source device can be reduced, and the size and weight thereof can be decreased.
Preferably, the inductor L includes a leakage inductor included in the transformer T.
Preferably, the leakage inductor of the transformer T is used as the inductor L. Thus, the number of components can be reduced. The cost of the switching power source device can be reduced, and the size and weight thereof can be decreased.
The advantages of various preferred embodiments of the present invention are summarized as follows.
When the peak value of the current flowing through the first winding reaches a predetermined value, the second switching device is turned on, and then the third switching device is turned on, and thereby the electrical signal can be increased in quantity. Accordingly, the first switching element Q1 can be turned on rapidly. The switching loss at turning off can be reduced, and increasing of the output current can be prevented.
The peak value of the current flowing through the first winding can be limited to a predetermined value, due to the peak current limiting circuit. Thus, magnetic saturation of the transformer can be prevented, and the high reliability of the switching power source device can be attained.
With the output voltage being reduced, the output power can be reduced, and the output current can be decreased, due to the on-period limiting circuit shortening the maximum on-period.
The device is operated in the mode in which starting and oscillation-stopping are repeated by appropriately setting the impedance of the turn-on delay circuit. Thus, the output power can be considerably reduced, and the shortcircuit current can be decreased. On the other hand, according to the examples 1 and 2, the output voltage current characteristic shown in FIG. 18 is obtained. The operation cannot be transitioned to the starting and stopping oscillation mode. The secondary current is increased at shortcircuiting. Thus, the rectification diode may be broken.
In the overcurrent protection circuit including the on-period limiting circuit for limiting the on-time using the time constant circuit, and the peak-current limiting circuit for limiting the peak current flowing through the first switching element Q1, the transistor Tr1 as the first switching device can also be used as the turn-off control of the first switching element Q1. Therefore, the number of components can be reduced. Moreover, the operation of the peak current limiting circuit and the operation by the on-period control circuit can be continuously switched over. The operation of the device can be stabilized. Furthermore, the transistors Tr2 and Tr3 may have a smaller current capacity compared to that of the transistor Tr1. Thus, rising costs can be avoided.
Even when the input voltage is changed, the output current (overcurrent point), at which the limitation of the peak current starts, can be kept constant. Therefore, magnetic saturation of the transformer can be reduced, and the size thereof can be decreased.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings.